1. Field of the Invention
This invention relates to sources of electrons and in particular, to such sources adapted for use in cathode ray tube devices, wherein a beam of electrons is modulated by a target or grid element therein.
2. Description of the Prior Art
There are a great variety of cathode ray tubes (CRT's), such as image conversion tubes, display storage tubes, information storage tubes and image correlation tubes, wherein a flow or beam of electrons is modulated by a grid or target element. For example, in a display storage tube, a video image is stored first upon a grid in the form of an array of discrete charges which serves to modulate or control a flood beam of electrons that is directed through the grid. The modulated flood beam of electrons then is directed onto a phosphor screen to provide thereby a visual display of the stored electron image. The grid of a storage display tube may be of large area and have a high transverse resistance. As a result, the grid may store a pattern of voltages thereon so that its control varies with position in the electron image. Significantly, the performance of such a display storage tube depends upon the energy distribution of the electron beam that is to be modulated or controlled by the grid. In particular, the current control characteristics, i.e. the electron beam current versus voltage of the modulating target or grid element, has a slope or transconductance g.sub.m that depends upon the energy distribution of the electron beam and the size of the holes or slots of the grid through which the electron beam is directed. This slope is a measure of the sensitivity of the storage display tube or other CRT device. For example, a grid or target element may have a pattern of voltage charges stored thereon which serves to control or modulate a beam of electrons. The sensitivity of the CRT incorporating such a grid or target element may be thought of in terms of the smallest voltage difference of the charge stored on the target element that may be detected by an electron beam. Typically, that incremental portions of the electron beam are modulated by the voltage imposed upon the CRT grid and the degree of modulation is dependent upon the width of the energy distribution of the electron beam. If the energy of the electron beam varies significantly, that is, its energy distribution is broad, then the degree of modulation, and hence the sensitivity of the CRT device, will be decreased. However, if the energy distribution of the electron beam is relatively narrow, then relatively small voltage differences are capable of controlling or modulating the electron beam, and the sensitivity of the CRT device is improved.
If the grid structure of the CRT device were ideal and the openings or slots therethrough of infinitesimally small dimension, the slope of the characteristic curve would depend only upon the energy distribution of that component of the electron velocity which is normal to the plane of the grid element, i.e. Z-axis velocity. Electrons in a beam may have velocity components along each of the X-, Y- and Z-axes. The normal or Z-component of electron velocity depends upon several factors. First, the electrons arriving at an equal potential region of the grid elements have differing energy components, because the electrons were emitted from the cathode element with a distribution of varying energies. For example, a thermionic cathode is heated to a predetermined temperature to emit electrons whose energy levels vary in a Maxwellian distribution, that is, a negative exponential distribution in energy; on the other hand, photocathode elements are excited by incident photons to emit electrons having varying energy levels in a Dewdney distribution. Thus, electrons emitted from either type of electron source arrive at the equal potential region of the grid element with differing energy levels since they were originally emitted from their cathode with a distribution of energy levels. Secondly, even though electrons are emitted with the same initial energy level, all electrons are not emitted in the same direction and are typically emitted in varying directions in accordance with a Lambertian distribution. Thus, since electrons having a velocity component transverse to the X-axis, must necessarily have a lower velocity component parallel to the Z-axis, i.e. the axis normal to the grid plane, the directional variation of electrons in a Lambertian spatial distribution results in a complicated Z-axis velocity distribution. Third, the electron lenses inserted between the cathode and grid elements change the distribution of the electron energy between the X- and Y-axis transverse velocities, and the Z-axis velocities, but cannot eliminate these distributions altogether. Therefore, the Z-axis distribution of electron velocity may be made larger than that distribution at the cathode surface.
Thus, it is highly desirable that the Z-axis velocity or energy distribution of the electron beam at the target or grid element be made more narrow than that energy distribution existing at the cathode element. Typical of the prior art, electron beams of limited energy spread have been obtained by means that eliminate both the low energy and the high energy components. For example, there are mass spectrometers that eliminate both the low and high energy components of the electron beam. While these devices are effective for a focused, concentrated beam of electrons, they have the disadvantage that the total current of the electron beam is severely limited. Elimination of the low energy component has little or no effect on the energy distribution of an electron beam emitted with a Maxwellian distribution from a thermionic cathode source. Further, the elimination of the low energy component of a flood beam of electrons emitted by a photocathode element with a Dewdney distribution, reduces the energy spread, but increases the ratio of the energy spread to the photocathode response so that the resulting efficiency is very low.
In the prior art, electron velocity selector structures have been suggested as in the article entitled, "Problem of Infrared Television-Camera Tubes versus Infrared Scanners", by J. A. Hall, Applied Optics, Vol. X, p. 838, April, 1971, and in the article by M. Auphan et al appearing in "Infrared Physics", Vol. 3, p. 117 (1963). As described in the aforementioned articles, a velocity selector structure has been used as a direct view image converter upon which infrared radiation (IR) is directed and whereby the non-infrared radiation or a portion thereof can be eliminated. More specifically, this device includes a photoconductive IR image layer to modulate the local potential of the elements of a locally excited matrix photocathode. The electron image corresponding to the input radiation image is directed to a phosphor screen, or camera tube target, under the control of an analyzer grid having a structure similar to that of a space charge tetrode. The voltage applied to the DC analyzer grid is adjusted to discard the DC value of the signal from the coolest or least emitting objects in the field of view. The construction of such a selector as suggested by these articles is undoubtedly too complex and expensive for many applications.